Benefits
Post-infection IBS with diarrhea: one unrepeated trial
In one 8-week trial in adults with post-infectious diarrhea-predominant IBS and a raised lactulose to mannitol ratio, 79.6% of those completing 5 g three times daily (15 g/day) improved by 50 points or more on a 500-point symptom score, versus 5.8% on a whey protein comparator. The gap is far larger than usual in IBS trials and has not been repeated. Only that group was tested; half of those screened had a normal test and were excluded.
Intestinal permeability: no overall change in pooled trials
A pooled analysis in hospital and disease populations found no overall change in permeability tests. Its dose subgroup is printed with contradictory numbers, so it gives no dose threshold, and no trial we found measured permeability at label doses in healthy adults. In exercise, three small crossover studies reported a smaller rise in a permeability marker after large doses, two later studies did not, and none showed fewer gut symptoms.
Sickle cell disease — FDA-approved indication
L-glutamine (branded as Endari®) is FDA-approved for sickle cell disease in patients ≥5 years old. In its pivotal trial, at 0.3 g/kg twice daily, median crises over 48 weeks were 3 versus 4 on placebo, median hospitalizations 2 versus 3, and median days in hospital 6.5 versus 11. The FDA label says the mechanism is not fully understood. The European regulator recommended refusal, judging that the same trial did not show effectiveness, and the application there was later withdrawn. Read this as a fact about a prescription medicine, not as a reason to buy a supplement. Endari is pharmaceutical-grade L-glutamine prescribed and monitored by a hematologist, the pivotal trial was funded by the manufacturer and has not been independently repeated, and about two thirds of patients in both groups were also on hydroxyurea. A tub of supplement-grade glutamine powder is not a substitute for it. Sickle cell disease is serious and nobody should self-treat it, stop or change a prescribed treatment, or copy the trial dose without their specialist.
Critical illness — caution required
A trial in 1,223 mechanically ventilated critically ill adults across 40 intensive care units found a trend to higher 28-day mortality with high-dose glutamine given by vein and feeding tube (32.4% versus 27.2%, P = 0.05 against a preset 0.044) and significantly higher in-hospital and 6-month mortality. The antioxidant arm of the same trial had no effect on mortality; in a later exploratory analysis the excess deaths sat in patients with kidney dysfunction, where antioxidants alone showed the same pattern. This reset prior enthusiasm for routine glutamine in ICU patients. European and American guidelines now caution against glutamine in shock and multi-organ failure. Burns were long treated as the surviving exception, but a separate trial in 1,209 patients with severe burns, who received glutamine through a feeding tube or by mouth, found no shortening of time to discharge alive and no improvement in survival, so that exception is now much weaker. Short bowel syndrome remains a specialist decision made by a treating team, not a self-care one.
Athletic recovery — overrated in marketing
Despite heavy marketing for muscle recovery, the trial evidence is weak. A meta-analysis that pooled 25 trials in physically active adults concluded that glutamine has no effect on aerobic performance, body composition or immune measures, and a six-week resistance training trial in 31 young adults found no extra gain in strength, torque or lean mass over placebo even at about 0.9 g per kilogram of lean tissue mass a day, far above a normal supplement serving. One small crossover in 16 people did report less soreness and faster strength recovery after eccentric exercise at about 0.3 g per kilogram per day, but a single crossover that size cannot carry the claim on its own. Glutamine stores in muscle are typically maintained adequately by dietary protein in well-fed athletes. For the gut during exercise, three small crossover studies (7 to 10 people) reported a smaller rise in a permeability marker after large doses, about 15 to 54 g for a 70 kg person; two later studies at about 20 g found no protection, one with a higher marker on glutamine, and none showed fewer gut symptoms.
Chemotherapy mucositis and short bowel syndrome
Used in oncology supportive care, with results that do not all point the same way. A meta-analysis of 15 randomized trials in 988 cancer patients found glutamine did not reduce how often mouth sores occurred but did reduce their severity and the number of severe cases, along with opioid use, feeding tube use and hospitalization. A separate meta-analysis of 6 trials in 441 patients with head and neck cancer found no clinical benefit from oral glutamine for radiation-induced mouth sores. Mouth-sore trials also often deliver glutamine as a rinse or a swish-and-swallow suspension aimed at the mouth lining, which is not the same exposure as swallowing a supplement drink. A prescription glutamine powder (30 g a day) was FDA approved for short bowel syndrome only alongside growth hormone and a specialized diet. In its trial the glutamine-only group improved least, and FDA now lists the product as discontinued. That is a prescription use managed by a treating team, not a consumer one. Short oral courses have not been shown to speed tumor growth, long-term data are lacking, and an intravenous transplant trial went the wrong way, so people in cancer treatment should ask their oncology team.
Wound healing and surgical recovery
Earlier and mostly small trials in burn and surgical patients reported faster wound healing and fewer infections, and a systematic review of 53 trials in 4,671 critically ill or major surgery patients did find fewer infectious complications with glutamine, though no reduction in deaths. That picture has since been undercut where it was strongest: a trial in 1,209 patients with severe burns found that glutamine by feeding tube did not shorten time to discharge alive and did not improve six-month survival. Almost all of this evidence comes from hospital nutrition support delivered by feeding tube or intravenously in seriously ill patients, which is not the same as swallowing supplement powder, and none of it validates glutamine for wound healing or skin appearance in otherwise healthy adults.
Sodium cotransport and oral rehydration
Glutamine functions as a sodium cotransport substrate in the small intestine, supporting fluid absorption parallel to glucose-mediated SGLT1. The absorption mechanism is real, but the hydration outcome is not. In rats given cholera toxin, both a free glutamine solution and an alanyl-glutamine dipeptide solution reduced sodium and water secretion, and in six healthy adults glutamine perfused directly into the jejunum through a tube roughly tripled water and sodium absorption. Where glutamine has been tested as something a person actually drinks, though, it has not delivered. A double-blind trial in 147 dehydrated children found a glutamine-based, glucose-free rehydration solution was no better than standard WHO oral rehydration solution for stool output, time to rehydrate, or the volume of solution needed. A hydration study in 19 healthy adults found a drink containing an alanyl-glutamine dipeptide held fluid better than plain water but no better than the same drink with electrolytes alone, and the authors concluded the electrolytes did most of the work. A prescription glutamine powder was FDA approved for short bowel syndrome only alongside growth hormone, which is a decision made by a treating team. On the evidence to date glutamine is not a hydration aid in its own right, which is why hydration is not listed as a use for it here.
Mechanism of action
Fuel for intestinal cells
Cells of the intestinal lining use glutamine as one of their fuels. Its share is known only from animals: about a third of the carbon dioxide from isolated rat intestine, and under a fifth in fed piglets, where glutamate from food was the larger fuel. We found no such percentage measured in people.
Conditionally essential under stress
Endogenous glutamine synthesis (primarily skeletal muscle) typically meets demand. Blood glutamine falls after major burns, major surgery and marathons, and in some ICU patients. A 1990 review proposed it may become conditionally essential in critical illness, but a low level is not a validated deficiency sign, and most patients tested in the largest ICU trial did not have one.
Immune cell substrate
Lymphocytes, macrophages, and neutrophils require glutamine at rates similar to glucose. Glutamine availability affects T-cell proliferation, cytokine production, and phagocytic function. This is the rationale behind trials of glutamine for preventing infection after surgery, and it remains a rationale rather than a demonstrated consumer benefit. The largest intensive care trial of high-dose glutamine found higher mortality, and in healthy, well-fed people glutamine supplementation has not improved immune measures.
Nitrogen carrier and acid-base balance
Glutamine is the major non-toxic transport form for nitrogen between tissues. Renal glutamine catabolism produces ammonia for excretion, contributing to acid-base homeostasis. Plays key role in interorgan amino acid metabolism.
Sickle cell: proposed redox mechanism
The FDA label for the prescription form says the mechanism in sickle cell disease is not fully understood. It notes that sickle red blood cells are more susceptible to oxidative damage and that glutamine may improve their NAD redox potential. This describes a prescription medicine, not a supplement effect.
Clinical trials
Double-blind randomized placebo-controlled trial of oral L-glutamine for post-infectious diarrhea-predominant IBS. Outcomes measured via the IBS Symptom Severity Score (IBS-SSS) — a validated, well-established instrument with a reduction of 50 points or more as the threshold for clinically meaningful improvement. Published in the journal Gut.
115 randomized, 106 analyzed; post-infectious IBS-D with a raised permeability test. 8 weeks.
Among those completing L-glutamine 5 g three times daily (15 g/day total) for 8 weeks, 79.6% (43 of 54) had an IBS-SSS fall of 50 points or more, vs 5.8% (3 of 52) on a whey protein comparator. Only completers were analyzed, the comparison group barely changed, and there was no follow-up after week 8. The authors called for larger trials; we found none reported as of October 2026.
Evidence review and pooled analysis of clinical trials evaluating L-glutamine supplementation for intestinal permeability outcomes (the 'leaky gut' framework). Included trials assessed permeability via lactulose-mannitol ratio or related markers. Published in Amino Acids. Subgroup analyses by dose, duration, and population.
10 to 11 trials in hospital and disease populations; no healthy adults, IBS or exercise studies.
Overall analysis showed no significant effect on permeability tests (WMD -0.00, 95% CI -0.04 to 0.03; I2 83.6%). Its dose subgroup is printed with numbers that contradict each other, so no dose threshold can be taken from it. The pooled trials were in hospital and disease populations, and no trial we found has measured permeability at label doses in healthy adults.
Phase 3 randomized double-blind placebo-controlled trial supporting FDA approval of L-glutamine (Endari®, Emmaus Medical) for sickle cell disease in 2017. Primary outcome: number of pain crises. Secondary outcomes: hospitalizations, acute chest syndrome, hospital length of stay. Published in NEJM.
230 patients ≥5 years old with sickle cell anemia or sickle β⁰-thalassemia. 48-week intervention.
With L-glutamine 0.3 g/kg twice daily, median sickle cell crises over 48 weeks were 3 versus 4 on placebo, median hospitalizations 2 versus 3, and median days in hospital 6.5 versus 11. Basis of the 2017 FDA approval and current use as a daily prescription therapy in sickle cell disease; the European regulator recommended refusal, judging that the same trial did not show effectiveness, and the application there was withdrawn in 2019. Three limits belong alongside that: the trial was funded by the manufacturer, Emmaus Medical, and has not been independently repeated; roughly two thirds of patients in both groups were also taking hydroxyurea, so the effect is an add-on to standard care; and nausea, noncardiac chest pain, fatigue and musculoskeletal pain were more common on glutamine. This is a prescription medicine used under specialist supervision, not a supplement regimen to copy.
Large multicenter randomized controlled trial evaluating high-dose glutamine and antioxidants in mechanically ventilated critically ill patients. 2×2 factorial design with parenteral and enteral glutamine supplementation. Primary outcome: 28-day mortality. Published in NEJM. Conducted across multiple ICUs in North America and Europe.
1,223 mechanically ventilated critically ill adults across multiple ICUs. ICU-stay-duration intervention.
In the glutamine comparison, 28-day mortality showed a trend to harm (32.4% versus 27.2%, p=0.05 against a preset 0.044), and in-hospital and 6-month mortality were significantly higher. The antioxidant factor had no effect on mortality (30.8% versus 28.8%, p=0.48). In a later exploratory analysis the excess deaths sat in patients with kidney dysfunction, and antioxidants on their own showed that pattern too. This finding reset prior enthusiasm for routine glutamine in critical care. Guidelines in Europe and North America now caution against glutamine in shock and multi-organ failure. The burns exception has since weakened as well: a separate trial in 1,209 patients with severe burns, who received glutamine through a feeding tube or by mouth, found no shortening of hospital stay and no improvement in survival.
Dose-response study evaluating oral L-glutamine for prevention of exercise-induced intestinal permeability — a known issue in endurance athletes that contributes to GI distress and 'leaky gut' biomarkers during prolonged exercise. Permeability assessed via lactulose-rhamnose or related dual-sugar absorption methods.
Ten recreationally active men, each completing four trials in a crossover design: a placebo trial and three glutamine doses taken two hours before a 60-minute treadmill run at 30 degrees Celsius.
Doses of 0.25, 0.5 and 0.9 g per kg of fat-free mass all lowered the lactulose to rhamnose permeability ratio versus placebo, with the largest reduction at the highest dose. The dosing basis is fat-free mass, not body mass, so the low dose is about 15 g and the high dose about 54 g for a 70 kg person with 15% body fat. Gut symptoms were low throughout and supplementation did not change them, and the results were reported as likelihoods rather than conventional significance tests in ten participants. Two later crossover studies at about 20 g found no protection, one of them with a higher permeability marker on glutamine.